This invention relates to the field of exploring for hydrocarbon deposits and more particularly to a method for indicating the proximity of a hydrocarbon deposit to a dry hole by means of measurement of thermal gradient in the dry hole.
It is generally well-known that heat flows from the inner portions of the earth to its surface in a generally uniform manner over the entire surface of the earth. Various measurements of the heat flow and variations in the heat flow at the earth's surface have been made in efforts to determine the structure of the earth's crust and even to explain the earth's origin. A detailed example of the attempts to measure the earth's heat flow by measurement of borehole temperatures and rock conductivity may be found in the publication entitled "Heat Flow and Precision Temperature Measurements in Boreholes" by John K. Costain and Phillip M. Wright, published by the SPWLA Tenth Annual Logging Symposium, May 25, 1969. The work described in this publication involves making precision temperature measurements in a borehole at a number of locations to determine the temperature gradient along the borehole. Conductivity of rock samples taken from the borehole was then measured under laboratory conditions. The gradient and conductivity values were then combined to estimate actual heat flow toward the earth's surface.
Another method of measuring heat flow in a borehole is illustrated by U.S. Pat. No. 3,808,889, issued to Rawson et al., on May 7, 1974. The Rawson device employs a solid-state thermopile incorporated into a logging sonde to provide electrical indications of the value of heat flowing through the device. This device is an example of apparatus for measuring heat flow and/or thermal gradient without the use of multiple temperature elements.
Another patent relevant to this field of interest is U.S. Pat. No. 3,864,969, issued to Smith, Jr., on Feb. 11, 1975. This device primarily is intended for measurement of earth formation thermal conductivity. The device includes means for coupling known quantities of heat to a station in the borehole and temperature-sensing elements for measuring the temperature rise caused by the heat. Thermal conductivities measured by the device are used to distinguish fresh-water-filled earth formations from hydrocarbon-filled earth formations. As noted in the background section of this patent, temperature logs taken by means of two high-resolution thermometers have been used in the past to detect casing leaks and fluid flow behind the casing. Such fluid flow almost always changes the temperature in the borehole from the normal formation temperature.
Thus it is seen that methods are known for measuring temperature, temperature gradient, thermal conductivity, and heat flow in a borehole but that such measurements have not been used in the exploration for hydrocarbon deposits missed by a borehole.